1. Field of the Invention
The present invention relates to a method for forming a resist mask pattern by light exposure which is to be used for manufacturing a semiconductor device or the like.
2. Description of the Prior Art
Referring to VLSI semiconductor devices which have recently been manufactured, a lot of transistors and wires are integrated in the order of submicron size on a silicon substrate. There has been used a light exposure technology in which a mask pattern is reduced (generally, by one fifth) and transferred to a photosensitive resin (resist) film which is provided on the silicon substrate so as to form a pattern of submicron size.
The minimum line widths of 1 Mbit and 4 Mbit DRAMs which have been mass-produced are set to 1.2 .mu.m and 0.8 .mu.m, respectively. Most exposure devices (steppers) for producing the 1 Mbit and 4 Mbit DRAMs utilize a bright line having a wavelength of 436 nm which is called a g-line and is emitted from an extra-high pressure mercury lamp. In some cases, there has been utilized a bright line having a wavelength of 365 nm which is called an i-line and is emitted from the extra-high pressure mercury lamp.
Referring to 16 Mbit and 64 Mbit DRAMS which will be produced in the future, it is expected that the minimum line widths will be set 0.6 to 0.5 .mu.m and 0.4 to 0.3 .mu.m. It is necessary to form resist masks having the minimum line widths so as to mass-produce the above-mentioned semiconductor devices. Consequently, it is required that a light exposure technology having higher resolution should be developed. It has been examined whether the i-line and EKISHIMA laser using krypton and fluorine which has a wavelength of 248 nm are utilized in place of the g-line so as to improve resolution by causing exposed light to have a smaller wavelength.
Referring to the exposure technology in which the exposure devices are utilized, however, the contrast of an optical image on the silicon substrate is deteriorated by the diffraction of light on pattern ends when a pattern size approximates a wavelength size. The resolution is much lower than threshold resolution as determined by an exposure wavelength and a numerical aperture NA of a lens provided on the exposure device.
To eliminate the above-mentioned drawbacks, there has been proposed a phase shifting method for inverting the phase of exposed light so as to improve the contrast of an optical image. The phase shifting method has been utilized exclusively for a mask technology.
By the phase shifting method, a pattern structure on a mask can be improved so as to enhance the contrast of the optical image and practical resolution. For this technology, there is used a mask which is called a phase shifting mask.
An example of the phase shifting mask is shown in FIG. 24. The reference numeral 41 denotes a quartz substrate transparent for an exposure wavelength. The reference numeral 43 denotes a thin chromium film which intercepts exposed light. The reference numeral 42 denotes a thin film (phase shifting film)transparent for the exposed light and having a thickness Ts. The thickness Ts has the following relationship with a refractive index n and a wavelength .lambda. of the exposed light. EQU Ts=.lambda./{2.multidot.(n-1)} (1)
The above-mentioned condition is set such that the phase of exposed light transmitted through the thin film 42 is shifted by a half wavelength (180 degrees). An aperture 49 always corresponds to the aperture of the mask. On the phase shifting mask are provided small apertures 40 along with the phase shifting films 42. The aperture 40 is not singly resolved therearound.
Referring to the phase shifting mask shown in FIG. 24, the phase of light transmitted through the aperture 49 is different from that of light transmitted through the apertures 40 by 180 degrees. Consequently, a light wave diffracted from the aperture 49 to its peripheral portion and a light wave from the apertures 40 cancel each other. As a result, light overflow can be controlled from the aperture 49 to its peripheral portion on an imaging plane so that the contrast of a projected image can be improved. Thus, the contrast of an image projected onto a resist is improved so that practical resolution can be enhanced. However, it is necessary to draw a first resist image for forming a chromium pattern and then draw a second resist image for specifying a phase shifting film by means of an exposure device using electron beams in order to fabricate the phase shifting mask shown in FIG. 24. It is required that electron beam drawing which takes a lot of time should be carried out twice. In addition, it is necessary to superpose a second pattern on a first pattern with high precision. Consequently, it is more difficult to fabricate the mask on a technical basis and costs are increased.
To solve the above-mentioned problems, there has been proposed a phase shifting mask shown in FIG. 25. The reference numeral 51 denotes a quartz substrate transparent for an exposure wavelength. The reference numeral 53 denotes a thin chromium film which intercepts exposed light. The reference numeral 52 denotes a resist film as a phase shifting film, of which film thickness Ts satisfies the formula (1) described above. The phase shifting mask is fabricated as follows. First, an aperture 59 is formed by a resist (not shown) by means of an exposure device using electron beams. Then, the chromium film 53 on the aperture 59 is removed by etching. Thereafter, the resist film 52 is provided as the phase shifting film and is exposed by far ultraviolet rays from the transparent substrate 51 side by using the chromium film 53 as a mask. Consequently, the aperture 59 is formed on the phase shifting film 52. Subsequently, the chromium film 53 provided under the phase shifting film 52 is etched so that an aperture 54 of the chromium film 53 is made larger than the aperture 59 of the phase shifting film 52. Consequently, there is formed an area 50 which is covered by the resist film 52 around the chromium film 53. The phase of light transmitted through the aperture 59 is different from that of light transmitted through the area 50 by 180 degrees. When the transmitted light are superposed, they cancel each other. Consequently, light overflow can be controlled from the aperture 59 to its peripheral portion. Accordingly, the distribution of light intensities on an image forming plane is made very sharp so that practical resolution can be enhanced. Referring to this method, the frequency of drawing executed by the exposure device using electron beams is not increased and the process of fabricating the mask is complicated. The resist film which remains as the phase shifting film is fragile so that it is difficult to carry out cleaning for removing refuse on the mask. Consequently, the above-mentioned method is of no practical use.
As described above, the phase shifting methods which have been proposed can be used for only a mask technique. The process of fabricating a mask is complicated. In addition, there is a great problem in respect of practicality. Consequently, it is desired that a technique for applying and using the phase shifting method should become more effective and simple.